There is a Wavelength Division Multiplexing (WDM) system that achieves high-speed communications by allowing simultaneous transmission of light of different wavelengths through an optical fiber.
An optical wavelength multiplexing apparatus forming the WDM system above includes a multiplexing/demultiplexing (MUX/DMUX) unit for optical wavelength multiplexing and demultiplexing and an amplifying (AMP) unit for optical amplification according to a transmission distance.
Also, for example, as depicted in FIG. 17, the MUX unit is provided with Variable Attenuator (VAT) control units (for example, λ1 to λ3) responsible for a light-level changing function for the purpose of equalizing light levels of wavelengths to be multiplexed.
Each VAT control unit has internal states including an ALC state for feedback control of adjusting the value of the optical attenuator so as to decrease a light attenuation amount to increase the output light level to a target value and an ALD state for control of adjusting the value of the optical attenuator so as to keep the light attenuation amount constant to decrease the output light level.
Furthermore, the ALC state is provided with a protection period in which the state does not make a transition to the ALD state, as a grace period for increasing the output light level to some degree after the state makes a transition from the ALD to the ALC state.
With reference to FIG. 18, the output light level and the transition of the internal state in the VAT control unit are explained below. As depicted in the drawing, in the VAT control unit in an ALD state, when a monitor value of the output light level exceeds a threshold for ALC transition, the VAT control unit causes the internal state to make a transition to an ALC state to start a protective-period operation.
Then, as depicted in the drawing, since the internal state is in the protection period of the ALC state, even when the output light level is below a threshold for ALD transition, the VAT control unit does not cause a transition to an ALD state, but performs feedback control of adjusting the value of the optical attenuator so as to decrease the light attenuation amount to increase the output light level to a target value.
Furthermore, as depicted in the drawing, when the internal state goes through the protection period of the ALC state and the monitor value of the output light level is below the threshold for ALD transition, the VAT control unit causes the internal state to make a transition to an ALD state and perform control of adjusting the value of the optical attenuator so as to keep the light attenuation amount constant to decrease the output light level.
As a technology associated with the conventional technology explained above, a method of controlling an optical branching and inserting switch for switching an arbitrary wavelength path at a node in an optical wavelength division multiplexing communication system is disclosed (see Japanese Laid-open Patent Publication No. 2007-67758).
However, the VAT control unit has the following problems.
That is, when a light input to a fiber is interrupted due to, for example, withdrawal of the fiber during the protection period of the ALC state, and then the fiber is again inserted (or a fiber is newly added) to recover the light input to the fiber (see (2) and (3) in FIG. 19), the VAT control unit is in an ALC state to continue decreasing the light attenuation amount and, consequently, a surge may occur.
Then, if the surge occurring in the VAT control unit is input to the AMP unit, the wavelength amplification amount of other existing channels is decreased and, consequently, problems arise such that a main-signal error in other channels or a breakdown of an AMP unit of the next node may occur.